Before I ordered the boards, I actually updated the schematic and board a little bit.

1. I added a 10 µF cap on the 5 volt rail to help with any potential inrush current needed by the relay.
2. I added a flyback diode parallel to the coil to protect the transistor (you can get reed relays with built-in flyback diodes, but I didn't manage to do that).
3. I moved the opto slightly further away from the sensor jack footprint.

The results of that are schematic and board set 1 in the Files section.

Unfortunately, the optoisolator footprint I chose was not a match for the optoisolators I have on hand, so I've had to order a suitable part and it won't be here until after the weekend. But the relay portion of the project does work: Setting GPIO 17 high closes the relay and releasing it opens it.

While the boards were being fabbed, I made a second version of the project. This one takes 12 volts in via a 2.1mm barrel connector and includes a 2A 5v power supply for the pi as well as using 12 volts as the supply for the opto diode (with a corresponding increase in the resistance of the series resistor). I'm probably just going to use a USB power brick as I have a ton of them, but it's an option.